Yeah, I think the sweet spot is (if you’ve got rank 5 all in one week), about 5350 merits/week to maintain rank 5, accounting for decay.
And because the decay of your merits isn’t visible (or at least, there’s no ingame way of seeing your next week merit count), so you have to over supply merits to guarantee you stay at that level…
That would be about 180 interdictions a week to maintain (still 30 merits/kill?), or quite a few garrison supplies (roughly 50 million Cr worth of supplies, if you fast track).
Rank 4 is much easier to maintain – once you’re there anyway 🙂